Conventionally, a latent image electrically or magnetically formed in an electrophotographic image forming apparatus or the like is developed with an electrophotographic toner (hereinafter may be referred to simply as “toner”). In electrophotography, for example, an electrostatic charge image (latent image) is formed on a photoconductor and then developed with a toner, thereby a toner image is formed. Typically, the toner image is transferred onto a transfer material such as a sheet, and then fixed on the transfer material such as a sheet. In a fixing step of fixing the toner image on a transfer sheet, heat fixing techniques such as a heating roller fixing technique and a heating belt fixing technique are commonly used, because of their high energy efficiency.
Recently, demands from the market for faster and more energy-saving operations of image forming apparatuses have been increasing, and toners that are excellent in low temperature fixability and capable of providing high-quality images have been requested. As a method for ensuring a toner low temperature fixability, there is a method of lowering the softening temperature of the binder resin of the toner. However, when the softening temperature of the binder resin is low, there are increased chances of so-called offset, in which a toner image partially adheres to the surface of the fixing members, and the adhered image transfers to a copy sheet (hereinafter also referred to as hot offset). Furthermore, heat resistant storage stability of the toner degrades, and there may occur so-called blocking, in which toner particles fuse with each other particularly under high-temperature conditions. In addition, there also occurs a problem in the developing device, that the toner melts and adheres to the internal portions of the developing device and the carrier to contaminate them, or there occurs a problem that it is more likely for the surface of a photoconductor to be filmed with the toner.
Using a crystalline resin as a binder resin of a toner is known as a technique that can solve these problems. A crystalline resin has a characteristic of rapidly softening from its crystalline state when it gets to the melting point. Therefore, it can lower the fixing temperature of the toner significantly while securing the heat resistant storage stability that is expressed at or below the melting point. That is, it can satisfy low temperature fixability and heat resistant storage stability at the same time at high levels. However, a crystalline resin having a melting point that allows low temperature fixability to be expressed is soft and susceptible to plastic deformation, although it is excellent in toughness. Therefore, simply using a crystalline resin as a binder resin results in a toner with a very poor mechanical durability, which causes various troubles in the image forming apparatus, such as deformation, aggregation, adherence, contamination of the members in the apparatus, etc.
Hence, there have conventionally been proposed many toners that use a crystalline resin and an amorphous resin in combination, as toners using a crystalline resin as a binder resin (see PTLs 1 to 5). They are better at satisfying low temperature fixability and heat resistant storage stability at the same time, than conventional toners made only of an amorphous resin. However, when the crystalline resin gets exposed on the surface of the toner, there occurs a problem that the toner particles aggregate due to stress of being stirred in the developing device, to constitute a cause of a white void. Therefore, this technique has not been able to take full advantage of a crystalline resin, because the additive amount of the crystalline resin should be limited.
There are also proposed many toners that use a resin in which a segment having crystallinity and a segment having an amorphous property are chemically bonded with each other. For example, there are proposed toners that use as a binder resin, a resin in which crystalline polyester and polyurethane are bonded with each other (see PTLs 6 and 7). There is proposed a toner that uses a resin in which crystalline polyester and an amorphous vinyl polymer are bonded with each other (see PTL 8). Further, there are proposed toners that use as a binder resin, a resin in which crystalline polyester and amorphous polyester are bonded with each other (see PTLs 9 to 11).
Furthermore, there are proposed a technique of adding inorganic fine particles to a binder resin made mainly of a crystalline resin (see PTL 12), and a toner that uses a crystalline resin having a cross-linked structure based on unsaturated linkage containing a sulfonic acid group (see PTL 13).
These proposed techniques are all excellent in satisfying low temperature fixability and heat resistant storage stability at the same time, but do not fundamentally remedy the softness attributed to the crystalline segment and cannot solve the problems related with the mechanical durability of the toner.
Moreover, as a major subject of the toners using a crystalline resin, there is a problem of scratch resistance of images. Because time is taken from when the toner melts on a fixing medium during heat fixing until when the crystalline resin in the toner gets recrystallized, the surface of the image cannot recover hardness quickly. Therefore, there occur problems that a scar is generated on the surface of the image or the glossiness changes, due to contact and sliding friction with a sheet discharging roller, a conveying member, etc. in a sheet discharging step after the fixing.
Further, when a resin in which a crystalline segment and an amorphous segment are chemically bonded with each other is used, the sharp melt property of the crystalline segment may not be maintained well, depending on the composition used and the linkage. Moreover, there is also a problem that the pigment tends to be located unevenly in such a resin, like in a crystalline resin.
Hence, it is currently requested to provide a resin for a toner, with which it is possible to obtain a toner that can satisfy low temperature fixability and heat resistant storage stability at the same time at high levels, and has excellent scratch resistance and excellent pigment dispersibility.